There are many applications for which precise control over the amount and/or rate at which a fluid is dispensed by a pumping apparatus is necessary. In semiconductor processing, for example, it is important to control the amount and rate at which photochemicals, such as photoresist chemicals, are applied to a semiconductor wafer. The coatings applied to semiconductor wafers during processing typically require a flatness across the surface of the wafer that is measured in angstroms. The rates at which processing chemicals are applied to the wafer has to be controlled in order to ensure that the processing liquid is applied uniformly.
Many photochemicals used in the semiconductor industry today are very expensive, frequently costing as much as $1000 a liter. Therefore, it is preferable to ensure that a minimum but adequate amount of chemical is used and that the chemical is not damaged by the pumping apparatus. Current multiple stage pumps can cause sharp pressure spikes in the liquid. For example, negative pressure spikes may promote out gassing and bubble formation in the chemical which may cause defects in wafer coating. Similarly, positive pressure spikes may cause premature polymer crosslinking which may also result in coating defects.
As can be seen, such pressure spikes and subsequent drops in pressure may be damaging to the fluid (i.e., may change the physical characteristics of the fluid unfavorably). Additionally, pressure spikes can lead to built up fluid pressure that may cause a dispense pump to dispense more fluid than intended or dispense the fluid in a manner that has unfavorable dynamics.
In particular, pressure spikes may be caused by the opening and closing of valves within the pumping apparatus. Thus, what is needed is a sequence for the opening and closing of valves within a pumping apparatus which minimizes or reduces pressure variations within the fluid.